This invention relates generally to color cathode ray tubes of the type having a shadow mask, and more particularly to structures for suspending a shadow mask in a color tube. The invention is especially concerned with an improved configuration of the stud comprising part of each of a plurality of mask suspension devices.
Conventional color cathode ray tubes have a glass envelope which comprises a flanged front panel sealed to a funnel. A shadow mask is supported adjacent to a phosphor screen pattern deposited on the inner surface of a faceplate portion of the front panel. The mask is supported on studs embedded in the inner surface of the front panel flange.
Proper tube operation requires that the shadow mask be suspended at a precise distance from the phosphor screen pattern and at a precise orientation relative thereto. Proper tube operation also requires that the phosphor screen pattern and the aperture pattern in the associated shadow mask be aligned with respect to the effective source of the electron beams in the assembled tube in the same way that they were aligned with respect to an effective point light source used to screen the phosphor pattern on the faceplate. If this corresponding relative alignment is not established, color purity errors will appear in the images displayed by the end-product tube.
This invention is especially useful when embodied in a novel type of shadow mask color tube, a portion of which is shown by FIG. 1. This novel tube 10 has a substantially rectangular, flangeless faceplate 12 on the inner surface of which is suspended a substantially rectangular shadow mask assembly 14. Shadow mask assembly 14 is illustrated as being of a low-cost, light weight, frameless, flexible character, being preferably formed integrally from a single sheet of electrically conductive material such as steel, all as described in detail and claimed in U.S. Pat. No. 3,912,963 assigned to the assignee of this invention.
Shadow mask assembly 14 is supported by four suspension devices, one at each corner of the mask. One such suspension device is indicated by 16. Suspension device 16 is comprised, in the illustrated embodiment, of a two-legged bracket 17 attached to a skirt 18 of shadow mask 14. Bracket 17, and the shadow mask assembly 14 it suspends are detachably engaged through spring 20 and lug 22 to stud 24. Stud 24 is embedded in the glass of faceplate 12. This suspension system is described in U.S. Pat. No. 3,890,526 assigned to the assignee of this invention.
This suspension arrangement has a number of unique requirements which are not imposed on conventional mask suspension systems. One of these requirements, and one which is the subject of this invention, is that any component of the suspension device which might be affixed in or to the phosphor-inner surface of faceplate 12 must not intrude upon the image area of the faceplate, nor interfere with faceplate screening operations. Interference with faceplate screening operations is illustrated by FIG. 2. Prior to screening, shadow mask assembly 14 is detached from faceplate 12. As illustrated, a plurality of metal studs 24, which are shown as being relatively flat with a cross-section of substantially greater width than thickness, remain embedded in faceplate 12. Several applications of screening fluids including three applications of phosphor screening fluids are normally required with each phosphor application comprising a mixture of one of the three primary color phosphors such as red, green, or blue, together with a photoresist sensitizing agent and an organic binder. Such a phosphor screening fluid is commonly referred to as a "slurry". To apply a screening fluid by using the radial flow suffusion process, faceplate 12 is rotated as the screening fluid is poured on the faceplate at the center area 13. As the faceplate 12 rotates, the fluid spreads to the edges of the panel and excess fluid is cast off from faceplate 12 by centrifugal force. For a more detailed description of such a process, see copending application Ser. No. 592,431 filed July 2, 1975.
As shown by FIG. 2, the free flow of the screening fluid may be intercepted, however, by the plurality of corner-located studs 24, whose flattish faces act to dam the flow. As a result, the radially out-rushing flow of screening fluid, indicated schematically by lines 26, "washes back," resulting in wave patterns 28 which tend to remain and be fixed as the screening fluid is dried by air and applied heat. The effect of this resulting non-uniformity in phosphor density is cumulative as faceplate 12 is successively screened by the application of other screening fluids. The wave patterns 28, which occur at all four corners of faceplate 12, are visible to the viewer.
A typical wave pattern 28 (considerably exaggerated for exemplary purposes) is shown in cross-section by FIG. 3. The radially out-rushing flow of screening fluid 26 in response to centrifugal force is shown by arrow 27. Upon contact with stud 24, the flow washes back into the image area, forming wave pattern 28. As each coating of screening fluid is applied, a progressive build-up of wave patterns 28 occurs. The deleterious effects of these wave patterns are three-fold. First, the thickened coatings are visible to the viewer as dark areas in the corners of the screen; second, cross-contamination of the color occurs; and third, under-exposure in the thickened area during the printing process results in non-adherence of the phosphor and consequent phosphor wash-off and flake-off.
An approach to resolving the problem of screening fluid washback during screening is shown by the configuration of stud 30 in FIG. 4. The provision of the legs 32 and 34 elevate the face 36 of stud 30. During screening of the faceplate, the screening fluid is suffused across the inner surface of the faceplate, passes between and around the legs 32 and 34 as shown by flow lines 38, and hence does not wash back onto the image area to cause visible wave patterns.
Another stud configuration (not shown) capable of inhibiting washback comprises a plurality of slender legs embedded in the faceplate, and supporting a bracket suspension means. This multi-legged configuration is considered impractical, however, in view of the difficulty in cleaning surplus, unattached screening fluid particles from between the legs. Such particles become loose and can migrate to gun electrode and cathode parts to cause arcing and emission problems.
Support studs having circular profiles have been commonly used to support shadow mask assembly in tubes having the conventional flanged faceplate. Such studs are embedded in the flange as shown, for example, by FIG. 3 of U.S. Pat. No. 3,005,921. In this flange location, the stud does not intrude upon the image area of the faceplate nor interfere with faceplate screening operations. If a stud of circular profile were embedded in the faceplate, however, the bluntness of the circular profile could result in appreciable washback, the effect of which could be visible to the viewer.